1. Field of the Invention
The present invention relates to a fixing apparatus favorably for use in an electrophotographic image forming apparatus, and to an image forming apparatus having the same.
2. Description of the Related Art
In an image formation using an electrophotographic manner, a photoreceptor charged to a uniform electric potential is exposed to light in accordance with image information so that an electrostatic latent image is formed. The formed electrostatic latent image is developed by a developer so as to be visualized. The visualized image is transferred on a recording paper or the like, and the transferred developer on the recording paper is made to be fixed so as to form a solid recording image.
The fixing apparatus used for such image formation, is generally composed of a heating roller and a pressure roller. The fixing apparatus has such a configuration that, in passing the recording paper on which the developer for forming a visualized image through a pressure-contact region (hereinafter referred to as a nip portion) of the heating roller and the pressure roller, which pressure-contact region is formed by pressing the pressure roller against the heating roller, unfixed developer is fused and fixed by heating of the heating roller and pressing of the pressure roller.
During a fixing operation in the fixing apparatus, there sometimes occurs a so-called hot offset that the developer fused on the nip portion of the both rollers is not all fixed on the recording paper, but a part of the developer is attached to a surface of the roller. For instance, the developer attached to the heating roller is transferred on a portion which should be properly a white base, on a recording paper on which the developer is to be subsequently fixed, with the result that an image defect is made to occur.
Moreover, on the pressure roller, the developer which has already fixed to a back surface of the conveyed recording paper, for instance as in a case of duplex print, is sometimes fused again by heat in passing through the nip portion and a part of the developer is transferred and attached to the pressure roller. The developer thus attached to the pressure roller may cause the image defect and further, may cause a soil of the back surface of the recording paper.
The image defect caused by the hot offset in the fixing apparatus sometimes remains, in a case of black-and-white print, mere defects such as a fog in a white base of the formed image, a soil on the back surface of the recording paper, or the like in a tolerable range. However, in a case of full-color print, since a developer having a color different from a prescribed one is transferred from the both rollers, there often occur practically intolerable defects.
In order to solve such problems, in a fixing apparatus, both a heating roller and a pressurizing roller, or only the heating roller is provided with a cleaning unit for cleaning a surface of the roller. FIG. 6 is a cross section view seen from the side, illustrating a simplified configuration of a conventional fixing apparatus 1.
The fixing apparatus 1 comprises a heating roller 2, a pressurizing roller 3, a temperature sensor 4 that detects a temperature of the heating roller 2, first and second paper peeling claws 5 and 6 that peel a recording medium such as a recording sheet winding around the heating roller 2 or the pressurizing roller 3 from the roller, and first and second cleaning rollers 7 and 8 that are the cleaning unit of the heating and pressurizing rollers 2 and 3.
The heating roller 2 has a heat source such as a heater 9 inside, and is heated and warmed by heat generated by the heater 9. A temperature at which the heating roller 2 heats is regulated by a control portion (not illustrated) that controls an operation of the heater 9 in accordance with a detection output of the temperature sensor 4. Toner of developer on a recording sheet is heated and melted by the warmed heating roller 2. The pressurizing roller 3 is placed so that a rotation axis thereof is parallel to a rotation axis of the heating roller 2 and so as to be pressed against the heating roller 2 and form a nip portion 10. The heating roller 2 and the pressurizing roller 3 are fixing rollers.
The first and second cleaning rollers 7 and 8 can rotate, and are disposed so as to be in contact with and slide on the heating and pressurizing rollers 2 and 3 (these rollers may be generically referred to as the fixing rollers), respectively. The first and second cleaning rollers 7 and 8 may rotate in a forward direction with respect to rotation directions of the heating and pressurizing rollers 2 and 3, or may rotate in a backward direction.
Here, the forward direction does not relate to a rotation direction around the axis but means a rotation direction such that surfaces of both the contacting rollers move in the same directions in an abutting area. The backward direction means a rotation direction such that the surfaces of both the contacting rollers move in the opposite directions, that is, pass each other in the abutting area.
The first and second cleaning rollers 7 and 8 slidingly rub the heating and pressurizing rollers 2 and 3 rotating with the cleaning members, thereby eliminating toner adhering to the surfaces of the fixing rollers and cleaning up the fixing rollers. However, toner that has been eliminated from the heating and pressurizing rollers 2 and 3 and has adhered and accumulated onto the first and second cleaning rollers 7 and 8 adheres to the heating and pressurizing rollers 2 and 3 again from the first and second cleaning rollers 7 and 8 while the first and second cleaning rollers 7 and 8 keep rotating and continue a cleaning operation, with the result that it becomes impossible to obtain a cleaning effect. The fixing apparatus 1 has a problem such that in order to obtain a good cleaning effect at all times, the first and second cleaning rollers 7 and 8 must be replaced with unused ones with extremely high frequency.
The fixing apparatus 1 as described above is appropriate to be installed in a so-called low-speed machine in which the number of recording sheets subjected to image formation per unit time is small. In a case where the fixing apparatus 1 is installed in the low-speed machine, an influence of heat extraction by recording sheets is small and a surface temperature of the heating roller 2 is kept at a set temperature that is appropriate for fixing, because an image formation processing speed is low and an interval of fed printing sheets is long. Accordingly, the residual toner adhering to the fixing rollers, especially, the heating roller 2 is securely brought into a molten state on the surface of the heating roller 2, with the result that the toner is fully cleaned up by the cleaning roller 7, and it is prevented that a recording sheet is stained.
These days, however, increase of an image formation processing speed in an image forming apparatus is required as increase of office work efficiency is required, and a so-called high-speed machine in which an image formation processing speed is as high as it can process 100-120 sheets per minute is in practical use. In the high-speed machine in which the number of sheets subjected to an image formation process per unit time is large, it is difficult to keep the surface temperature of the heating roller at the set temperature appropriate for fixing, because a large number of recording sheets pass by the surface of the heating roller warmed by set consumption of electric power and a large amount of heat is extracted. Therefore, the fixing apparatus 1 using the cleaning rollers has a problem such that in a case where the fixing apparatus is installed in the high-speed machine, it is impossible to obtain a sufficient cleaning effect.
In a prior art for solving such problems, a belt-shaped cleaning member called a web sheet is used, whereby continuous supply of the cleaning member is enabled to seek the maintaining of cleaning performance, and a length of the cleaning member abutting on the fixing roller in the rotation direction of the roller is increased to seek increase of the cleaning effect (refer to Japanese Unexamined Patent Publication JP-A 2003-107952).
FIG. 7 is a cross section view seen from the side, illustrating a simplified configuration of a conventional fixing apparatus 11 using the belt-shaped cleaning member. The fixing apparatus 11 is similar in basic configuration to the fixing apparatus 1 illustrated in FIG. 6, but is configured so as to be capable of matching the high-speed machine. That is to say, in order to deal with heat extraction by recording sheets and keep the fixing rollers at the set temperature, a heating roller 12 is provided with two heaters 20a and 20b, and a pressurizing roller 13 is provided with a heater 21. Accordingly, as the temperature sensors, a first temperature sensor 14 that detects a temperature of the heating roller 12 and a second temperature sensor 15 that detects a temperature of the pressurizing roller 13 are disposed. Moreover, the first cleaning unit 18 that has the belt-shaped cleaning member is provided for the heating roller 12 between a first paper peeling claw 16 and the first temperature sensor 14 with respect to a rotation direction of the heating roller 12, and the second cleaning unit 19 that has the belt-shaped cleaning member is provided for the pressurizing roller 13 between a second paper peeling claw 17 and the second temperature sensor 15 with respect to a rotation direction of the pressurizing roller 13.
Since the first and the second cleaning unit 18 and 19 are configured so as to be symmetrical with respect to a recording sheet 23 passing through a nip portion 22 that is a pressure-contact area of the heating roller 12 and the pressurizing roller 13, a configuration of the first cleaning unit 18 will be described as a representative.
The first cleaning unit 18 includes a wound belt-shaped cleaning member 24 called a web sheet, a feed-out roller 25 that sends out the cleaning member 24, a take-up roller 26 that winds up the cleaning member 24 fed out from the feed-out roller 25, and a pressure-contact roller (may be referred to as the web pressure-contact roller) 27 disposed between the feed-out roller 25 and the take-up roller 26 so as to press the cleaning member 24 against the heating roller 12. Although driving means for driving the feed-out roller 25 and the take-up roller 26 is also included in the first cleaning unit 18, illustration thereof is omitted here.
The cleaning unit 18 and 19 press the cleaning members 24 against the heating and pressurizing rollers 12 and 13 (the fixing rollers) rotationally driven in directions of arrows 28 and 29, respectively, in a state where the take-up rollers 26, the feed-out rollers 25 and the pressure-contact rollers 27 are not rotated and are kept still, and make the fixing rollers and the cleaning members 24 slidingly rub each other, thereby eliminating molten toner adhering to circumferential faces of the fixing rollers, and storing the eliminated toner in an almost molten state into gaps formed by the cleaning members 24 located between the pressure-contact rollers 27 and the take-up rollers 26 and the surfaces of the fixing rollers.
When the toner stored in the gaps reaches a certain amount, the cleaning unit 18 and 19 cause the take-up rollers 26 to perform a take-up operation in a direction of arrow 30 to take up the cleaning members 24 by a predetermined length, and make the toner leave from the surfaces of the fixing rollers in a state where the toner remains adhering to the cleaning members 24.
According to the cleaning unit 18 and 19 using the cleaning members 24, it is possible to feed out unused parts of the cleaning members 24 from the feed-out rollers 25 and recover cleaning ability, at a point of time that the cleaning members 24 have cleaned up a certain amount of toner. Therefore, it is possible to avoid complicatedness of frequent replacement of the cleaning members. Moreover, it is possible to increase the length of the cleaning member 24 abutting on the fixing roller in the rotation direction of the fixing roller, by pressing the cleaning member 24 by the pressure-contact roller 27, with the result that it is possible to obtain a good cleaning effect in the high-speed machine in which fixing on a recording sheet is executed with high frequency.
However, both the fixing apparatus 1 and the fixing apparatus 11 using the belt-shaped cleaning member 24 have a problem as follows. In general, oil is applied to the fixing rollers of the fixing apparatuses 1 and 11 in order to promote releasability of a recording sheet on which a toner image has been fixed from the rollers.
In a case where the cleaning rollers as in the fixing apparatus 1 are used, the oil is applied by a method such as pump up the oil from, for example, an oil tank to supply to the surfaces of the cleaning rollers and apply the oil to the surfaces of the fixing rollers from the surfaces of the cleaning rollers (for example, refer to Japanese Unexamined Patent Publication JP-A 9-106210 (1997)). Moreover, in a case where the belt-shaped cleaning members as in the fixing apparatus 11 are used, the oil is applied by a method such as impregnate the oil into the cleaning members in advance and press the cleaning members against the fixing rollers by the pressure-contact rollers, thereby applying the oil to the surfaces of the fixing rollers (for example, refer to Japanese Unexamined Patent Publication JP-A 9-101729 (1997)).
The oil applied to the surfaces of the fixing rollers is heated and softened by the warmed fixing rollers, thereby existing as thin films on the surfaces of the fixing rollers, and promoting releasability of a recording sheet on which a toner image has been fixed from the rollers. However, no matter which of the oil application methods described above is adopted, an “excessive oil phenomenon” that the oil applied to the surfaces of the fixing rollers becomes slightly excessive often occurs. For example, in a case where the cleaning members impregnated with the oil in advance are used, in an initial state where unused parts are fed out from the feed-out rollers and used for cleaning, the amount of the oil seeping out of the cleaning members when the cleaning members are pressed against the fixing rollers by the pressure-contact rollers is large, and becomes slightly excessive.
A state where the oil on the fixing rollers is excessive will be described below, taking the fixing apparatus 11 as an example. FIG. 8 is a view for describing a state where a mixture of the toner and the oil accumulates in the vicinity of tip portions of the paper peeling claws 16 and 17 in the fixing apparatus 11.
When the oil on the fixing rollers becomes excessive, and the toner heated and melted by the fixing rollers, especially, the heating roller 12 mix the oil, whereby slipperiness of the mixture becomes extremely high. Accordingly, even if the mixture of the residual toner on the heating roller 12 and the oil rotationally moves together with the heating roller 12 while adhering to the heating roller 12 and enters the pressure-contact area of the heating roller 12 and the cleaning member 24, the oil contained in the mixture and the oil impregnated into the cleaning member 24 cause slipping, and the mixture slips through, with the result that a phenomenon such that the mixture cannot be eliminated by the cleaning member 24 and cleaning performance decreases occurs.
The mixture having slipped through the pressure-contact area of the cleaning member 24 and the heating roller 12 rotationally moves more together with the heating roller 12, and part of the mixture shifts to the pressurizing roller 13 in the pressure-contact area (the pressure-contact area when a recording sheet does not exist) of the heating roller 12 and the pressurizing roller 13. The mixtures of the toner and the oil adhering to the heating roller 12 and the pressurizing roller 13, respectively, rotationally move in accordance with rotation of the heating and pressurizing rollers 12 and 13, and the mixtures are scraped by tip portions 16a and 17a abutting on the rollers of the first and second paper peeling claws 16 and 17 disposed to the heating roller 12 and the pressurizing roller 13, respectively, and accumulate as mixture lumps 31 and 32 in the vicinity of the tip portions 16a and 17a. 
After the recording sheet 23 on which a toner image has been formed in an image forming portion of an image forming apparatus is sent to the fixing apparatus 11 and passed through the nip portion 22 to be subjected to fixing, the recording sheet is ejected from the nip portion 22 in a direction, for example, along the circumferential face of the heating roller 12, and peeled from the heating roller 12 by the first paper peeling claw 16. The recording sheet 23 peeled from the heating roller 12 is further conveyed along the first paper peeling claw 16, but when the recording sheet ejected from the nip portion 22 of the fixing rollers reaches a certain length, a part around a tip portion 23a of the recording sheet 23 in a conveying direction falls downward in the vertical direction (a direction of arrow 33) due to a weight of the recording sheet 23 itself.
At this moment, a part of the recording sheet 23 located in the vicinity of the tip portion 16a of the first paper peeling claw 16 moves toward the tip portion 16a of the first paper peeling claw 16 in reaction when the tip portion 23a of the recording sheet 23 falls downward, and comes in contact with the mixture lump 31 accumulated in the vicinity of the tip portion 16a of the first paper peeling claw 16, with the result that a stain like a small dot or like a comet leaving a long thin trail is formed on the surface of the recording sheet 23. Such a stain is formed also in a case where the recording sheet 23 is ejected along the pressurizing roller 13.
Thus, the conventional fixing apparatuses have a problem such that when the oil supplied in order to promote releasability of a recording sheet on which a toner image has been formed from the fixing roller is excessively supplied, the mixture of the molten toner and the oil accumulates on the fixing roller, especially, in the vicinity of the tip portion of the paper peeling claw and makes a stain on the recording sheet.